Two species of wild caught kangaroo rat and the laboratory gerbil will be examined for behavioral and physiological changes resulting from short periods of malnutrition. Malnutrition induced polydipsia has been recently identified in several desert rodents. Given their well developed body water balance mechanisms an explanation for a pattern of over-drinking during food shortage is important. Of initial concern is whether the increase in water consumption noted during food restriction in a laboratory setting occur under conditions that approximate natural habitat. "Pseudo-deserts" will be constructed with sand and photo-periods that match sun and moon appearances at the capture sites. Deviation in the distribution of drinking behavior during the day-night cycle will be closely monitored with seeds present and during periods of partial food restriction and total deprivation. Additional experiments will test the predictive ability of a proposed model of malnutrition induced rodent drinking. The respective contributions of elevated body fluid osmolality and extracellular fluid volume decline to malnutrition drinking will be evaluated. Changes in electrolyte concentrations and urea-nitrogen levels will be used to evaluate adjustment in protein metabolism during malnutrition. The proposed research program is considered to be an initial effort toward the long term goal of understanding human malnutrition, the contribution of body fluid deviations to deaths resulting from malnutrition, and the elucidation of procedures that may prevent such deaths. It should be emphasized that animals utilized in these investigations will not be subject to terminal food deprivation. The longest food deprivation duration will be four days, a period carefully selected on the basis of previous research to maximize research findings but minimize animal discomfort.